Epigenetic silencing of CD4 in T cells committed to the cytotoxic lineage

The process of thymocyte development culminates in the maturation of helper (CD4+) and cytotoxic (CD8+) T cells from their common precursors, the CD4+CD8+ double-positive cells. A crucial step during lineage specification is the termination of expression of either the CD4 or the CD8 coreceptor. A silencer element within the first intron of the CD4 gene is sufficient for CD4 transcriptional repression in cells of the cytotoxic lineage, as well as in thymocytes at earlier stages of differentiation. Here we show that the function of the CD4 silencer is required only at distinct stages of development. Its deletion before the initiation of lineage specification resulted in CD4 derepression throughout thymocyte differentiation. By contrast, once cells committed to the cytotoxic CD8+ lineage, the CD4 locus remained silent through subsequent mitoses, even when the silencer element was excised. The epigenetic inheritance of the silenced CD4 locus was not affected by the inhibition of DNA methylation or histone deacetylation, and may thus involve other mechanisms that ensure a stable state of gene expression.

Requirement for RORgamma in thymocyte survival and lymphoid organ development

Most developing thymocytes undergo apoptosis because they cannot interact productively with molecules encoded by the major histocompatibility complex. Here, we show that mice lacking the orphan nuclear hormone receptor RORgamma lose thymic expression of the anti-apoptotic factor Bcl-xL. RORgamma thus regulates the survival of CD4+8+ thymocytes and may control the temporal window during which thymocytes can undergo positive selection. RORgamma was also required for development of lymph nodes and Peyer's patches, but not splenic follicles. In its absence, there was loss of a population of CD3-CD4+CD45+ cells that normally express RORgamma and that are likely early progenitors of lymphoid organs. Hence, RORgamma has critical functions in T cell repertoire selection and lymphoid organogenesis.

Function of the chemokine receptor CXCR4 in haematopoiesis and in cerebellar development

Chemokines and their receptors are important in cell migration during inflammation, in the establishment of functional lymphoid microenvironments, and in organogenesis. The chemokine receptor CXCR4 is broadly expressed in cells of both the immune and the central nervous systems and can mediate migration of resting leukocytes and haematopoietic progenitors in response to its ligand, SDF-1. CXCR4 is also a major receptor for strains of human immunodeficiency virus-1 (HIV-1) that arise during progression to immunodeficiency and AIDS dementia. Here we show that mice lacking CXCR4 exhibit haematopoietic and cardiac defects identical to those of SDF-1-deficient mice, indicating that CXCR4 may be the only receptor for SDF-1. Furthermore, fetal cerebellar development in mutant animals is markedly different from that in wild-type animals, with many proliferating granule cells invading the cerebellar anlage. This is, to our knowledge, the first demonstration of the involvement of a G-protein-coupled chemokine receptor in neuronal cell migration and patterning in the central nervous system. These results may be important for designing strategies to block HIV entry into cells and for understanding mechanisms of pathogenesis in AIDS dementia.

Cre-loxP-mediated gene replacement: a mouse strain producing humanized antibodies

BACKGROUND

The bacteriophage-derived Cre-loxP recombination system operates efficiently in mammalian cells. This system is particularly useful in gene-targeting experiments in the mouse, and has already been used to generate 'clean' deletions of target genes in the germ line, as well as to inactivate target genes in a conditional manner (based on regulated expression of the Cre recombinase). In principle, Cre-loxP-mediated recombination should also allow gene replacement, and thus the introduction of virtually any kind of mutation into the genome.

RESULTS

We used the Cre-loxP system, in mouse embryonic stem cells, to replace the mouse gene C gamma 1, which encodes the constant region of the heavy chain of IgG1 antibodies, with its human counterpart. The mutation was transmitted through the mouse germ line, and the resulting mutant mice were crossed with mice expressing kappa light chains with a human, instead of a mouse, constant region. Mice homozygous for both mutations produce humanized, kappa-chain-bearing IgG1 antibodies at the same level and efficiency as wild-type mice produce murine IgG1 antibodies. These animals should enable the ex vivo production of humanized, chimeric monoclonal antibodies specific for any antigen to which the mouse can respond.

CONCLUSIONS

Cre-loxP-mediated gene replacement is a simple and efficient general method of targeted mutagenesis in the mouse.

Generation of a mouse strain that produces immunoglobulin kappa chains with human constant regions

Humanized antibodies are highly efficient as immunotherapeutic reagents and have many advantages over rodent antibodies. A mouse strain was generated by gene targeting to replace the mouse kappa light chain constant (C) region gene with the human C kappa gene. Mice homozygous for the replacement mutation (C kappa R) produced normal concentrations of serum antibodies, most of which carry chimeric kappa light chains, and mounted normal immune responses to hapten-protein conjugates. This technology provides a feasible option for the generation of high-affinity humanized antibodies by means of the powerful somatic hypermutation-selection mechanism.

Independent control of immunoglobulin switch recombination at individual switch regions evidenced through Cre-loxP-mediated gene targeting

We have employed a method based on the Cre-loxP recombination system of bacteriophage P1 to generate a mouse strain in which the JH segments and the intron enhancer in the IgH locus are deleted. By analysis of immunoglobulin isotype switch recombination in heterozygous mutant B cells activated by lipopolysaccharide plus interleukin-4, we show that, on the mutant chromosome, switch recombination at the mu gene switch region is strongly suppressed, whereas the switch region of the gamma 1 gene is efficiently rearranged. These data demonstrate an independent control of switch recombination at individual switch regions and suggest that, in the process of switch recombination, the alignment of the recombining strands occurs independently of and probably after the introduction of double-strand breaks into the switch regions involved.

Deletion of the immunoglobulin kappa chain intron enhancer abolishes kappa chain gene rearrangement in cis but not lambda chain gene rearrangement in trans

Immunoglobulins (Ig) secreted from a plasma cell contain either kappa or lambda light chains, but not both. This phenomenon is termed isotypic kappa-lambda exclusion. While kappa-producing cells have their lambda chain genes in germline configuration, in most lambda-producing cells the kappa chain genes are either non-productively rearranged or deleted. To investigate the molecular mechanism for isotypic kappa-lambda exclusion, in particular the role of the Ig kappa intron enhancer, we replaced this enhancer by a neomycin resistance (neoR) gene in embryonic stem (ES) cells. B cells heterozygous for the mutation undergo V kappa-J kappa recombination exclusively in the intact Ig kappa locus but not in the mutated Ig kappa locus. Homozygous mutant mice exhibited no rearrangements in their Ig kappa loci. However, splenic B cell numbers were only slightly reduced as compared with the wild-type, and all B cells expressed lambda chain bearing surface Ig. These findings demonstrate that rearrangement in the Ig kappa locus is not essential for lambda gene rearrangement. We also generated homozygous mutant mice in which the neoR gene was inserted at the 3' end of the Ig kappa intron enhancer. Unexpectedly, mere insertion of the neoR gene showed some suppressive effect on V kappa-J kappa recombination. However, the much more pronounced inhibition of V kappa-J kappa recombination by the replacement of the Ig kappa intron enhancer suggests that this enhancer is essential for V kappa-J kappa recombination.

Gene targeting in the Ig kappa locus: efficient generation of lambda chain-expressing B cells, independent of gene rearrangements in Ig kappa

The production of lambda chain-expressing B cells was studied in mice in which either the gene encoding the constant region of the kappa chain (C kappa) or the intron enhancer in the Ig kappa locus was inactivated by insertion of a neomycin resistance gene. The two mutants have similar phenotypes: in heterozygous mutant mice the fraction of lambda chain-bearing B cells is twice that in the wildtype. Homozygous mutants produce approximately 7 times more lambda-expressing B cells (and about 2.3 times fewer total B cells) in the bone marrow than their normal counterparts, suggesting that B cell progenitors can differentiate into either kappa- or lambda-producing cells and do the latter in the mutants. Whereas gene rearrangements in the Ig kappa locus are blocked in the case of enhancer inactivation, they still occur in that of the C kappa mutant, although in this mutant RS rearrangement is lower than in the wildtype. This indicates that gene rearrangements in the Ig lambda locus can occur in the absence of a putative positive signal resulting from gene rearrangements in Ig kappa, including RS recombination. Complementing these results, we also present data indicating that in normal B cell development kappa chain rearrangement can be preceded by lambda chain rearrangement and that the frequency of kappa/lambda double producers is small and insufficient to explain the massive production of lambda chain-expressing B cells in the mutants.